The present invention relates to power inverter controller for transformerless solar PV systems.
Renewable distributed energy resources (DERs), such as solar photovoltaic (PV) and wind power systems, have been getting more attention recently to be used as alternatives to fossil fuels. PV power systems are considered as one of the most attractive renewable DER technologies thanks to the abundance of solar energy and the declining capital and operational expenses. Generally, PV systems can be interfaced with the utility grid through transformer-isolation or transformerless configurations. Since line frequency transformers are heavy, inefficient, and cost-ineffective for PV systems, transformerless configurations are attracting more and more interests from both research and commercial points of view. However, the lack of galvanic isolation in the transformerless configurations will lead to a common-mode (CM) leakage current between the PV panels and the ground through parasitic capacitors, which reduces the overall efficiency and grid current quality and may cause serious electromagnetic interference and insecurity issues. The parasitic capacitance is approximately 60 nF to 110 nF every kilowatt of the PV array. Therefore, various inverter topologies with specific modulation strategies have been introduced to suppress the leakage current, in which only a few topologies have been developed into industrial products, e.g., H5, H6, and HERIC inverters. The H5 structure is adopted by the SMA Solar Technology due to its simple topology with the least number of switches.
FIG. 1 illustrates a typical grid-connected transformerless PV system using an H5 inverter, where iLeak stands for the CM leakage current, CP is the PV parasitic capacitor mentioned previously. In order to extract the maximum power of a PV array under different ambient conditions (irradiance and temperature), maximum power point tracking (MPPT) algorithms, such as Perturb & Observe (P&O) and Incremental Conductance (In-Cond), are employed to control the power-electronics stage. Although there are numerous existing methods to implement MPPT for grid-tied PV systems, most of them use two-stage cascaded DC/DC-DC/AC converting systems or single-stage DC/AC inverters with PI-based controllers or their variants. These methods may suffer from one or multiple of the following major drawbacks:                PI-based controllers require iterative tuning efforts when system parameters change;        It is relatively difficult to find optimal gain and time constants for the controllers;        Extra pulse width modulation (PWM) modules are required;        Some of the methods require multiple stages of costly converters, which reduces the converting efficiency; and        CM leakage current is not considered in most methods.        
The topology of an H5 inverter is similar to the single-phase full-bridge inverter by adding an extra DC-bypass switch “S5” that disconnects the PV array from the utility grid during the current-freewheeling periods. FIG. 1 shows the topology of H5 inverter with the leakage current (iLeak) between the PV array and the ground. In general, there are four operation modes for H5 inverters, which are depicted FIG. 2. The first operation mode (FIG. 2(a)) is the active mode which occurs during the positive-half cycle, where the switches S1, S4, and S5 are conducting and the current flows through S1 and S5 and then returns to the cathode of the PV array through S4. The second mode of operation shown in FIG. 2(b) is also referred to as the current freewheeling mode with the zero-voltage vector. In this mode, S1 is triggered on, while S4 and S5 are turned-off. The current is conducting through the freewheeling diode of S3. FIG. 2(c) illustrates the third mode of operation of H5 inverter, which is the active mode that occurs during the negative-half cycle. During mode 3, switches S2, S3, and S5 conduct and the current flows through the inductors L1 and L2 in the opposite direction of that in mode 1. The fourth mode is the freewheeling mode during the zero-voltage vector where S2 and S5 are turned-off and S3 is on. Similar to S3 in mode 2 (FIG. 2(b)), S1 works as a freewheeling diode in mode 4. Table I and FIG. 3 show the operation modes and space vector modulation (SVM) of the H5 inverter.
TABLE IH5 INVERTER SWITCHING STATESModeS1S2S3S4S5Vout110011VPV2100000301101−VPV4001000